Thermal recording print head and method for manufacturing same

ABSTRACT

A thermal print head and a method of manufacturing it using a thermally stable support mandrel and a plurality of spaced turns of copper-coated resistance wire wrapped on the mandrel. The turns are held on the mandrel by slabs of potting compound retained in under-cut grooves on the mandrel while the connections to the print elements are made by slitting the turns along a longitudinal groove in the mandrel. The copper coating is removed to expose the resistance wire at the desired printing element locations.

United States Patent 1191 Lane, III

[ THERMAL RECORDING PRINT HEAD AND METHOD FOR MANUFACTURING SAME [75]Inventor: Charles Lane, III, Meadowbrook,

[73] Assignee: Honeywell Inc., Minneapolis, Minn.

[22] Filed: Apr. 3, 1974 21 Appl. No.: 457,622.

[52] US. Cl. 219/216, 346/76 R [51] Int. Cl. H05b 1/00 [58] Field ofSearch..... 346/76 R, 139 C; 219/216 [56] References Cited UNITED STATESPATENTS 2/1970' Zissen 219/543 x 2/1970 Alexander et al. 219/216 1 Jan.21, 1975 Primary Examiner-C. L. Albritton Attorney, Agent, orFirm-Arthur H. Swanson;

Lockwood D. Burton; Mitchell J. Halista 57 ABSTRACT A thermal print headand a method of manufacturing it'using a thermally stable supportmandrel and a plurality of spaced turns of copper-coated resistance wirewrapped on the mandrel. The turns are held on the mandrel by slabs ofpotting compound retained in under-cut grooves on the mandrel while theconnections to the print elements are made by slitting the turns along alongitudinal groove in the mandrel. The copper coating is removed toexpose the resistance wire at the desired printing element locations.

8 Claims, 5 Drawing Figures 5/1971 Colello ..2l9/216 Patented Jan. 21,1975 THERMAL RECORDING PRINT HEAD AND METHOD FOR MANUFACTURING SAMEcRoss REFERENCE TO RELATED APPLICATIONS Subject matter shown but notclaimed herein is shown and claimed in a divisional application ofCharles e. Lane, 111 having Ser. No. 490,230, filed on July 22, 1974BACKGROUND OF THE INVENTION mounting on a printed circuit board. Thermalprint heads may be found in Class 219 Electric Heating, Subclass 216 ofthe United States Patent Office classification system.

2. Description of the Prior Art In order to obviate the-problemsassociated with con.- ventional recording techniques in recordinginformation, particularly from data processing systems, thermalrecorders have been developed for recording on a thermally responsiverecording medium. For example, such thermal recorders obviate the use ofan inking pen which type of pen is undesirable for many recorderapplications because of problems associated with clogging of thepen aswell as the need to replenish the ink supply. Similarly, the thermalrecorder is free of the problems associated with a high voltage type ofrecording wherein recording electrodes using an electric arc to mark arecording medium are exposed to arc erosion while the arc imposeslimitations on the environmental exposure of the recording apparatus,e.g., an explosive atmosphere'Further, the thermal recorder can be usedfor recording analog and digital information on the same recordingmedium. Conventional thermal recording devices are shown in US Pat. Nos.3,145,071; 3,312,979; 3,334,352; 3,354,817; 3,453,647; and 3,718,564.While these prior art thermal recorders are capable of recording on athermally responsive medium, noneof the aforesaid recorders employs arecording head which is capable of being assembled by an automaticmanufacturing technique. Further, a practical thermal print head mustlend itself for use with printed circuits to minimize the manufacturingcosts thereof and to produce a miniaturized print head capable ofoperating as a high speed printing element, i.e.,

a light weight printing head having minimal thermal inertia, or heatingand cooling time.

SUMMARY OF THE INVENTION Accordingly, it is an object of the presentinvention I to provide an improved thermal print head adaptable forautomatic manufacturing techniques while exhibiting high speed recordingcapabilities in a miniaturized assembly.

Another object of the present invention is to provide va novel method ofmanufacturing a thermal print head.

section support mandrel. A longitudinal undercut groove is located ineach face of the support mandrel adjacent to this first longitudinaledge with a first free end of each of the heating elements passingacross a first one of the longitudinal grooves and a second free end ofeach of the heating elements passing across a second one of thelongitudinal grooves. A retainer means is located within each of thegrooves and is arranged to encompass the portion of the heating elementspassing across a corresponding groove to retain the heating elements onthe first longitudinal edge and to fill the corresponding longitudinalgroove. The print head is manufactured by a method including the stepsof winding a resistance heating wire having an electrically conductivecoating with spaced turns on the mandrel with each turn of the wirepassing across the first longitudinal support edge of the mandrel toform a thermal printing element at the apex of the first longitudinalsupport edge, and moulding a slab of retaining material around theportions of the wires passing over the undercut grooves while fillingthe undercut groove by the retaining material to be retained therein.The wire turns are out along a third longitudinal groove located in athird face of the mandrel to form free wire ends for each printingelement suitable for providing a connection to energizing circuits forthe print head.

BRIEF DESCRIPTION OF THE DRAWING A better understanding of the presentinvention may DESCRIPTION OF THE PREFERRED EMBODIMENT DETAILEDDESCRIPTION Referring to FIGS. 1 to 4 in more detail, there is shown asequential manufacturing operation embodying the present invention forproducing a thermal recording head having a plurality of thermalrecording elements in an integral arrangement. Starting with FIG. 1,there is shown a thermally stable support mandrel 2 having a triangularcross-section. A pair of longitudinal undercut grooves, or dovetails, 4and 6 are provided in adjacent faces 2A and 2B, respectively, of thesupport mandrel 2 which faces 2A and 2B join along one edge to define alongitudinal support edge 8. A third face 2C of the mandrel 8 is alsoprovided with a longitudinal groove 10 suitable for attaching thethermal print head to a support member, as discussed hereinafter. Thesupport mandrel 8 may be an aluminum bar which is hard-anodized andcoated with a suitable material, e.g., tetrafiuorethylene, to provideelectrical insulation for the aluminum base member. The mandrel 8 iswound with a single layer of spaced turns of an electrically insulatedand copper plated resistance wire 12 with the spaced turns of thewinding passing across the first longitudinal support edge 8. This wireconsists of a core of resistance wire which is copper plated and coatedwith solder-through, i.e., heat-meltable, electrical insulation toenable the resistance wire to be connected to a support member, asdiscussed hereinafter.

As shown in FIG. 3, a wire retaining member in the form of a slab of apotting compound 14, 16 is moulded in place on the outside surface ofeach of the faces 2A and 28 having the aforesaid undercut longitudinalgrooves 4 and 6, respectively, therein. This potting compound 14, 16 isarranged to have the turns of the resistance wire 12 embedded thereinwhereby to retain thewires on the mandrel 2-in-a fixed spacedrelationship while the portion of the potting compound extending intothe undercut grooves 4 and 6 provides a means for locking the pottingcompound 14, 16 to the mandrel 2. Subsequently, each of the turns of theresistance wire 12 is cut by passing acutter (not shown) along thegroove located in the third face 2C of the mandrel 2. This cuttingoperation is effective to provide a pair of free ends for each of thesegments of the resistance wire passing across the aforesaid firstlongitudinal support edge 8, as shown in FIG. 4.

The completed thermal print head is attached to a support member, suchas a printed circuit board "18, by self tapping' attaching screws 20passing through the circuit board 18 into the longitudinal groove 10 inthe third face 2C of the mandrel 2. The width and depth of the groove 10is selected to provide suitable space for the self-tapping attachingscrew 20. The free ends of the resistance wire heating elements areattached to plated-through holes 22 in the printed circuit board 18 byany suitable soldering technique which removes the solder-throughinsulation from the wires and connects the copperplating to theplated-through holes 22. Each of the plated'through holes 22 is, inturn, connected to printed wiring 24 located on the printed circuitboard 18 for conducting an energizing signal to a corresponding one ofthe'thermal printing elements and may, further, be connected to otherassociated equipment located on the printed circuit board 18 to minimizeextraneous electrical connections.

The exposure of the core of the resistance wires forming-each of theprinting areas at the apex of the first longitudinal support edge 8 maybe achieved either before or after attaching the thermal print head tothe printed circuit board 18 by etching the wires to a predetermineddistance on either side of the first support edge 8. This etchingprocessusing, for example, a solution of ferric chloride, is arranged toremove the solderthrough coating and the copper plating on theresistance wires while not affecting the electrical insulation of thesurface of the mandrel 2. Thus, the resistance wires will heat up onlyin the area where the copper plating has been removed since the copperplating will serve as a conductor along other portions of the resis-'tance wires to bypass the internal core of resistive material. Thus, theamount of material to be heated to form a thermal print characterislimited to the etched area on the first longitudinal support edge 8.

Accordingly, it may be seen that there has been provided, in accordancewith the present invention, a novel thermal print head and a novelmethod for producing the thermal print head in an automaticmanufacturing operation to produce aplurality of thermal printingelements in a integral construction.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A thermal print head comprising:

a thermally stable and electrically insulating support mandrel meanshaving a first longitudinal support edge between a pair of mandrelfaces;

a plurality of spaced heating elements supported on said first supportedge;

plurality of electrical conductors electrically connected to saidheating elements respectively with each heating element having a pair ofelectrical conductors attached thereto with each electrical conductor ofeach pair of said electrical conductors extending across and in contactwith a corresponding one of said mandrel faces, and

retaining means for individually attaching each of said electricalconductors to said mandrel and for maintaining said spaced relationshipof said heating elements.

2. A thermal print head as set forth in claim 1 wherein said retainingmeans includes an undercut longitudinal groove in each of said adjacentmandrel faces and a resilient slab filling each of said grooves whileencompassing corresponding ones of said plurality of electricalconductors.

3. A thermal print head as set forth in claim 2 wherein said heatngelements each includes a segment of a resistance wire passing acrosssaid support edge.

4. A thermal print head as set forth in claim 3 wherein said pluralityof electrically conductors are each an electrically conductive coatingon said resistance wire.

5. A thermal print head as set forth in claim 4 wherein said mandrel hasa triangular cross-section.

6. A thermal print head as set forth in claim 5 and further including aprinted circuit board having printed wiring thereon connected torespective ones of said electrical conductors.

7. A thermal print head as set forth in claim 1 wherein said supportmandrel means includes a support mandrel having an electricallyinsulating coating covering the outside surface of said support mandrel.

8. A print head comprising;

a thermally stable and electrically insulating support mandrel meanshaving a first longitudinal support edge between a pair of mandrelfaces;

a plurality of spaced printing elements supported on said first supportedge;

a plurality of electrical conductors electrically connected to saidprinting elements respectively with each printing element having a pairof electrical conductors attached thereto with each electrical conductorof each pair of said electrical conductors extending across acorresponding one of said mandrel faces, and

retaining means for individually attaching each of said electricalconductors to said mandrel and for maintaining said spaced relationshipof said heating elements. I

1. A thermal print head comprising: a thermally stable and electricallyinsulating support mandrel means having a first longitudinal supportedge between a pair of mandrel faces; a plurality of spaced heatingelements supported on said first support edge; a plurality of electricalconductors electrically connected to said heating elements respectivelywith each heating element having a pair of electrical conductorsattached thereto with each electrical conductor of each pair of saidelectrical conductors extending across and in contact with acorresponding one of said mandrel faces, and retaining means forindividually attaching each of said electrical conductors to saidmandrel and for maintaining said spaced relationship of said heatingelements.
 2. A thermal print head as set forth in claim 1 wherein saidretaining means includes an undercut longitudinal groove in each of saidadjacent mandrel faces and a resilient slab filling each of said grooveswhile encompassing corresponding ones of said plurality of electricalconductors.
 3. A thermal print head as set forth in claim 2 Wherein saidheatng elements each includes a segment of a resistance wire passingacross said support edge.
 4. A thermal print head as set forth in claim3 wherein said plurality of electrically conductors are each anelectrically conductive coating on said resistance wire.
 5. A thermalprint head as set forth in claim 4 wherein said mandrel has a triangularcross-section.
 6. A thermal print head as set forth in claim 5 andfurther including a printed circuit board having printed wiring thereonconnected to respective ones of said electrical conductors.
 7. A thermalprint head as set forth in claim 1 wherein said support mandrel meansincludes a support mandrel having an electrically insulating coatingcovering the outside surface of said support mandrel.
 8. A print headcomprising; a thermally stable and electrically insulating supportmandrel means having a first longitudinal support edge between a pair ofmandrel faces; a plurality of spaced printing elements supported on saidfirst support edge; a plurality of electrical conductors electricallyconnected to said printing elements respectively with each printingelement having a pair of electrical conductors attached thereto witheach electrical conductor of each pair of said electrical conductorsextending across a corresponding one of said mandrel faces, andretaining means for individually attaching each of said electricalconductors to said mandrel and for maintaining said spaced relationshipof said heating elements.